Protecting my circuit, flip flop, AND gate, etc.

Discussion in 'General Electronics Chat' started by kballing, Jul 6, 2007.

  1. kballing

    Thread Starter Member

    Jun 18, 2007
    18
    0
    So I have never really built a circuit before but have a decent understanding of how most things work. I am putting together a circuit in which I use a D type toggle flip flop controlled by a pushbutton to control an AND gate, in conjunction with another pushbutton. This AND gate output resets my toggle and controls another IC which controls a 12V solenoid valve. For instance, I saw this website, http://www.doctronics.co.uk/4013.htm, that shows a very simple circuit and this one, http://www.cpemma.co.uk/flipflop.html, that has a lot of extra stuff to prevent bouncing. So my question is, what do I need to take into account as far as decoupling, debouncing, pull down resistors, etc. to sufficiently protect my circuit from killing itself?
     
  2. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Hi...I will draw some stuff and post a PDF later tonight for you.....Daniel.
     
  3. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Hi...

    I hope this will help, or give you some ideas.....

    Daniel.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    I might add, if you are using a logic circuit to control a solenoid (aka relay), it is a good idea to use protective diodes.
     
  5. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Just another few notes....

    You can use logic and micro's to do almost anything these days, and because of this they are used in almost every enviroment....
    There is a lot to know about protection rite from the power supply, through the inputs to the outputs....
    I can remember being on a "casting of non-ferous metals course at Chelmsford college, and visiting the electronics department, there.....
    A group of students were working on a CNC mill at the time, and the control circuit was all in TTL....
    They were having all sorts of problems with glitches and spikes.....
    The problem was that there was not a decoupling capacitor in sight on any IC.... say no more!!!

    I will post some tip's and info later.....

    Daniel.
     
  6. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    There is a very good application note about grounding and decoupling
    by Paul Brokaw called "An IC Amplifier User's Guide to Decoupling,
    Grounding,. and Making Things Go Right for a Change". It is at

    http://www.analog.com/UploadedFiles/Application_Notes/135208865AN-202.pdf

    For switch debouncing checkout "Examining switch-debounce circuits"
    by Ron Mancini at http://www.edn.com/article/CA196470.html

    (* jcl *)
     
  7. kballing

    Thread Starter Member

    Jun 18, 2007
    18
    0
    Thanks for all the helpful responses. I'm attaching the schematic of my circuit. This is the first circuit that I have ever really had to design myself so I'm expecting some errors. I someone would please look over it and tell me if it looks all right.
     
  8. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    I would use a protective diode near the solenoid valves, to protect the transistor from the reverse surges created by the solenoids. Also, the two resistors are not necessary there. The transistor that controls the solenoids needs a base resistor in order to not over saturate. As a rule of thumb, the base current should be 1/10 of the collector current. That NPN transistor is not to be used with the emitter connected to the solenoid valves, but the emmiter should be connected to the earth and the collector to the solenoid valves. The other two transistors are also connected in a wrong manner. It seems that this circuit has too much unnecessary components, including the transistors that aren't controlling the solenoid valves.

    I don't have time to analyse the circuit to search more errors, but have these pointed.
     
  9. kballing

    Thread Starter Member

    Jun 18, 2007
    18
    0
    thanks cumesoftware for the criticism. I've made a lot of revisions but I'm sure I'm sure there are still a lot of problems. I'm attaching a revision, I'd appreciate any more comments.
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    The output section for the solenoid seems to be good now. I assume that you used true logic in that output, that is, a logic high will cause the transistor to conduct, right?

    I also see that you taken out those transistors, since TTL outputs are suffice to drive 10 TTL inputs. Your circuit is still confusing, since there are resistors that won't do nothing (except comsuming power). It is a good idea to use a grounding resistor at the input of the AND gate that has the switch. You don't need any resistor at the other input since the 74LS74 can sink currentby itself. Actually the resistor will degrade the sourcing capabilities of the 74LS74. I think there are more resistors needed to be taken out.

    Just keep in mind that a TTL output doesn't need a resistor to drive a TTL input, even if the output is an open collector output (TTL inputs will go to Vcc per se, when allowed to float).

    Also, one rule of thumb to turn your diagram less confusing: represent Vcc on top, GND on bottom, and current pathway going top to bottom. Moreover, in the case of having two sections of a circuit, one above the other in the sheet, you don't need to represent Vcc all the way up and GND all the way down across the sheet, just repersent for each section Vcc on top of that section and GND on the bottom of that section.
     
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